The present invention relates to a power supply circuit, a driver circuit, an electro-optical device, an electronic instrument, and a common electrode drive method, and the like.
As a liquid crystal display (LCD) panel (display panel in a broad sense; electro-optical device in a broader sense) used for electronic instruments such as portable telephones, a simple matrix type LCD panel and an active matrix type LCD panel using a switching element such as a thin film transistor (hereinafter abbreviated as “TFT”) have been known.
The simple matrix method can easily reduce power consumption as compared with the active matrix method. On the other hand, it is difficult to increase the number of colors or display a video image using the simple matrix method. The active matrix method is suitable for increasing the number of colors or displaying a video image, but has difficulty in reducing power consumption.
The active matrix type LCD panel is driven so that the polarity of the voltage applied to a liquid crystal (electro-optical material in a broad sense) forming a pixel is reversed alternately. In this case, the voltage level applied to a pixel electrode forming a pixel can be reduced by changing a common electrode voltage (common voltage) supplied to a common electrode opposite to the pixel electrode at the inversion drive timing, whereby power consumption can be reduced.
When driving the active matrix type LCD panel, a high power supply voltage is required for a gate line for selecting the pixel, and a low power supply voltage is required for a source line for supplying a grayscale voltage to the pixel. These power supply voltages are generated by boosting a system power supply voltage by a charge-pump operation which can be realized at low power consumption. For example, power consumption can be further reduced by increasing the cycle of the charge-pump operation when generating a voltage for low-load applications. The high power supply voltage applied to the gate line is generated by the charge-pump operation of which one cycle is two lines (two horizontal scan periods), for example.
However, the boost voltage generated by the charge-pump operation changes in synchronization with the cycle of a charge-pump signal for performing the charge-pump operation. In JP-A-2004-252022, the cycle of the subfield is set to be an integral multiple of the cycle of the charge-pump signal, for example. This enables a horizontal-striped display unevenness appearing in each subfield to be spatially dispersed, whereby the display unevenness in one frame can be eliminated.
A National Television Standards Committee (NTSC) video signal (television signal in a broad sense) is known as a terrestrial analog color television signal. An output operation using the NTSC video signal is necessary when outputting an image and sound using a cathode ray tube (CRT) device. In recent years, a portable electronic instrument (e.g. digital still camera (DSC)) equipped with an LCD panel has also been required to display an image on the LCD panel using the NTSC video signal.
The NTSC video signal is designed so that the number of horizontal scan periods (number of scan lines) within one vertical scan period alternately becomes an even number and an odd number in frame units. On the other hand, a driver circuit drives an LCD panel on the assumption that the number of scan lines in each frame is identical. Therefore, when generating the high power supply voltage of the gate line in a two-line cycle, the boost voltage for generating the common electrode voltage changes every two lines, whereby the voltage of the common electrode changes. This causes a flickering phenomenon, whereby the display quality deteriorates.
According to the technology disclosed in JP-A-2004-252022, when the number of scan lines in each frame differs (e.g. television signal), the number of timings of the charge-pump operation in each frame (number of edges of charge-pump signal) differs. Therefore, the amount of change in the voltage of the common electrode differs depending on the frame, whereby the voltage applied to the liquid crystal changes depending on the frame. This causes a flickering phenomenon, whereby the display quality deteriorates.